1. Field
Embodiments described herein generally relate to methods and apparatus for forming an electrode structure used in an energy storage device. More particularly, embodiments described herein relate to methods and apparatus for characterizing nanomaterials used in forming electrode structures for energy storage devices.
2. Description of the Related Art
Electrical energy can generally be stored in two fundamentally different ways: 1) indirectly in batteries as potential energy available as chemical energy that requires oxidation and reduction of active species, or 2) directly, using electrostatic charge formed on the plates of a capacitor.
An electric battery is a device that converts chemical energy into electrical energy, typically consisting of a group of electric cells that are connected to act as a source of direct current. Generally, a cell consists of two dissimilar substances, a positive electrode and a negative electrode, that conduct electricity, and a third substance, an electrolyte, that acts chemically on the electrodes. The two electrodes are connected by an external circuit (e.g., a piece of copper wire); the electrolyte functions as an ionic conductor for the transfer of the electrons between the electrodes. The voltage, or electromotive force, depends on the chemical properties of the substances used, but is not affected by the size of the electrodes or the amount of electrolyte.
Batteries are classed as either dry cell or wet cell. In a dry cell the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing. In a wet cell the electrolyte is in liquid form and free to flow and move. Batteries also can be generally divided into two main types—rechargeable and nonrechargeable, or disposable. Disposable batteries, also called primary cells, can be used until the chemical changes that induce the electrical current supply are complete, at which point the battery is discarded. Disposable batteries are most commonly used in smaller, portable devices that are only used intermittently or at a large distance from an alternative power source or have a low current drain. Rechargeable batteries, also called secondary cells, can be reused after being drained. This is done by applying an external electrical current, which causes the chemical changes that occur in use to be reversed. The external devices that supply the appropriate current are called chargers or rechargers.
A battery called the storage battery is generally of the wet-cell type; i.e., it uses a liquid electrolyte and can be recharged many times. The storage battery consists of several cells connected in series. Each cell contains a number of alternately positive and negative plates separated by the liquid electrolyte. The positive plates of the cell are connected to form the positive electrode; similarly, the negative plates form the negative electrode. In the process of charging, the cell is made to operate in reverse of its discharging operation; i.e., current is forced through the cell in the opposite direction, causing the reverse of the chemical reaction that ordinarily takes place during discharge, so that electrical energy is converted into stored chemical energy. The storage battery's greatest use has been in the automobile where it was used to start the internal-combustion engine. Improvements in battery technology have resulted in vehicles in which the battery system supplies power to electric drive motors instead.
Typically, ordinary capacitors store a small amount of charge generally due to their size and thus only store a small amount of electrical energy. In an effort to form an effective electrical energy storage device that can store sufficient charge to be useful as independent power sources, or supplemental power source for a broad spectrum of portable electronic equipment and electric vehicles, devices known as electrochemical capacitors have been created. Electrochemical capacitors are energy storage devices which combine some aspects of the high energy storage potential of batteries with the high energy transfer rate and high recharging capabilities of capacitors. The term electrochemical capacitor is sometimes described in the art as a super-capacitor, electrical double-layer capacitors, or ultra-capacitor. Electrochemical capacitors can have hundreds of times more energy density than conventional capacitors and thousands of times higher power density than batteries. It should be noted that energy storage in electrochemical capacitors can be both Faradaic or non-Faradaic. Energy storage in conventional capacitors is generally non-Faradaic, meaning that no electron transfer takes place across an electrode interface, and the storage of electric charge and energy is electrostatic.
In both the Faradaic and non-Faradaic electrochemical capacitors, capacitance is highly dependent on the characteristics of the electrode and electrode material. Ideally, the electrode material should be electrically conducting and have a large surface area.
To make electric batteries and electrochemical capacitors more of a viable product it is important to reduce the production cost, and improve the efficiency of these types of devices. While previous advances have allowed these batteries to meet the needs of the past, much more drastic changes must be made to meet the needs of the future. More specifically, the charge storing electrode (anode) must be made with much greater energy density. However, theoretical limits on the carbon anodes used today have essentially been reached. This means drastic changes must be made, and taking full advantage of the prospering nanofabrication industry as well as the latest breakthroughs in anode research is essential to meet the needs of the future.
However, simple structures and mechanics change significantly when analyzed at the nanoscale (less then 1−6 meters), yielding conventional metrology obsolete. Therefore, there is a need for metrology methods of characterizing nanoscale materials used for forming electric batteries and electrochemical capacitors.